(a) Field of the Invention
This invention relates to a curtain (blade) bound preventing device adapted to focal plane shutters of a type formed so that both front curtain (front blade) and rear curtain (rear blade) may be in an exposure aperture covering position in the cocked and uncocked states of the shutter.
(b) Description of the Prior Art
This type of focal plane shutter is excellent in respect of being able to effectively prevent light leakage. Its conventional example shall be briefly explained in the following with reference to FIGS. 1A to 1E. In FIG. 1A, symbol P denotes a shutter base plate having an exposure aperture Pa, reference numeral 1 denotes a front blade operating arm which is pivoted on the base plate P and has a pin 1a and hook portions 1b and 1c, 2 denotes a spring for biasing the front blade operating arm 1 clockwise, 3 denotes a front blade driving arm which is pivoted concentrically with the front blade operating arm 1 on the base plate P and has a pin 3a, 4 denotes a spring for biasing the front blade driving arm 3 counterclockwise, 5 denotes a connecting lever which is pivoted on the front blade driving arm 3 and has a bent portion 5a engageable with the hook portion 1c, 6 denotes a spring for biasing the connecting lever 5 counterclockwise, 7 denotes a front blade locking lever which is pivoted on the base plate P and has a bent portion 7a engageable with the hook portion 1b, 8 denotes a spring for biasing the front blade locking lever 7 counterclockwise. 9 denotes an X-contact lever which is pivoted on the base plate P and has a bent portion 9a engaged with the side wall of the front blade operating lever 1, 10 denotes a spring for biasing the X-contact lever 9 counterclockwise, 11 denotes a rear blade operating arm which is pivoted on the base plate P and has a pin 11a and hook portion 11b, 12 denotes a spring for biasing the rear blade operating arm 11 clockwise, 13 denotes a release lever which is pivoted on the base plate P and has a bent portion 13a engageable with the hook portion 11b, 14 denotes a spring for biasing the release lever 13 clockwise, 15 denotes a rear blade driving arm which is pivoted concentrically with the rear blade operating arm 11 on the base plate P and has a curved portion 15a and pin 15b, 16 denotes a spring for biasing the rear blade driving arm 15 counterclockwise, 17 denotes a rear blade locking lever which is pivoted on the base plate P and has a bent portion 17a, engageable with the tip portion of the rear blade driving arm 15, 18 denotes a spring for biasing the rear blade locking lever 17 counterclockwise and 19 denotes a pin which is erected on the base plate P and is engageable with the connecting lever 5. Here, the springs 4 and 10 are so selected as to be stronger than the spring 2 and the spring 16 is so selected as to be stronger than the spring 12.
In FIGS. 1B to 1E, symbol P.sub.1 denotes an intermediate plate which is secured at a predetermined spacing on the base plate P and has an aperture aligned with the exposure aperture Pa, P.sub.2 denotes a cover plate which is secured at a predetermined spacing on the intermediate plate P.sub.1 and has an aperture aligned with the exposure aperture Pa, A denotes a front blade group which is arranged in a space sectioned by the intermediate plate P.sub.1 and cover plate P.sub.2 and consists of a plurality of opaque laminae and B denotes a rear blade group which is arranged in a space sectioned by the base plate P and intermediate plate P.sub.1 and consists of a plurality of opaque laminae. The front blade group A is operatively connected with the pin 1a of the front blade operating arm 1 and is so arranged as to be able to take the respective illustrated positions in response to the rotating position of the front blade operating arm 1. The rear blade group B is operatively connected with the pin 11a of the rear blade operating arm 11 and is so arranged as to be able to take the respective illustrated positions in response to the rotating position of the rear blade operating arm 11.
FIG. 1A shows a cocked state of the shutter. In this state, when the shutter is released, first the release lever 13 will be rotated counterclockwise. By this rotation of the release lever 13, the rear blade operating arm 11 will be rotated clockwise to the position wherein the pin 11a engages with the curved portion 15a of the rear blade driving arm 15 and will move the rear blade group B to the position in FIG. 1C from the position in FIG. 1B. That is to say, the rear blade group B will be moved from the exposure aperture Pa covering position to the opening position so as to be ready for the subsequent exposing operation. In the next stage of the shutter release, the front blade locking lever 7 will be rotated clockwise to release the front blade operating arm 1. In the cocked state of the shutter, as the front blade operating arm 1 is connected with the front blade driving arm 3 through the connecting lever 5, when the front blade operating arm 1 is released, said arm 1 will be rotated counterclockwise integrally with the front blade driving arm 3 by the force of the spring 4. At this time, the X-contact lever 9 will be also rotated counterclockwise following the front blade operating arm 1 by the force of the spring 10. By this counterclockwise rotation of the front blade operating arm 1, the front blade group A will be moved from the position in FIG. 1C to the position shown in FIG. 1D and will open the exposure aperture Pa to start the exposure. In the final stage of this shutter opening stroke, the connecting lever 5 will collide with the pin 19 and will be rotated clockwise, therefore the front blade operating arm 1 will separate from the front blade driving arm 3 and the counterclockwise rotating motion of the front blade operating arm 1 will be ended. In this case, the front blade operating arm 1 will tend to be rotated clockwise by the force of the spring 2 but will be prevented by the X-contact lever 9 from this rotation and will therefore remain the front blade group A in its opening position. Thereafter, when a proper exposure time elapses, the rear blade locking lever 17 will be rotated clockwise by a well known exposure controlling means to release the rear blade driving arm 15. When the rear blade driving arm 15 is thus released, said arm 15 will be rotated counterclockwise together with the rear blade operating arm 11 by the force of the spring 16. Therefore, the rear blade group B will move from the position in FIG. 1D to the position shown in FIG. 1E and will close the exposure aperture Pa to end one exposure. In the final stage of this counterclockwise rotation of the rear blade driving arm 15, the X-contact lever 9 will be rotated clockwise to the illustrated position by said arm 15 and therefore, following it, the front blade operating arm 1 will be rotated clockwise to the illustrated position by the force of the spring 2. As a result, the front blade group A will move from the position in FIG. 1D to the position shown in FIG. 1E to cover the exposure aperture again. Thus, one exposure operation will be all completed, the front blade operating arm 1 will engage with the front blade locking lever 7 and the rear blade operating arm 11 will come to engage with the release lever 13. The shutter is cocked by simultaneously clockwise rotating the front blade driving arm 3 and rear blade driving arm 15 to the illustrated positions by means of a well known cocking member not illustrated engaged with the pins 3a and 15b.
As apparent from the above explanation, in this kind of conventional shutter, when the pin 11a of the rear blade operating arm 11 collides with the curved portion 15a of the rear blade driving arm 15 when the shutter is released, the rear blade operating arm 11 will once bound and then engage with the rear blade driving arm 15. This means that the rear blade group B will once retreat from the exposure aperture Pa and then again advance into the exposure aperture Pa. Therefore, this bounding phenomenon will result in an inaccurate exposure time to be controlled and the reduction of the durability of the shutter. As a result, there are defects that the time interval from the time of releasing the shutter to the time of starting the actual exposure must be designed to be comparatively long and that the motion speed of the blade itself can not be made high.
There is already suggested a method wherein such elastic body as of rubber is provided in the part to be collided with to prevent such bounding phenomenon. However, in such method, not only it is difficult to fix the blade starting position but also the variation with the lapse of time is likely to occur. It is not preferable.